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  <title>Document</title>
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  <!-- <script>
    // new set(里面放可迭代对象)
    // var set = new Set('abcdefghijklmnopqrstuvwxyz')
    // set.add(9)
    // set.add(15)

    // console.log(set)

    let obj = {
      cc: 5
    }
    console.log(5 in obj)
  </script> -->
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